This invention relates, generally, to optical instruments such as spectrometers and, more particularly, to a method and apparatus for calibrating such instruments.
Optical instruments such as spectrometers use light to perform various spectral analyses. Typically, a light beam, after being filtered by a monochrometer, interferometer and Fourier transform, scanning filter photometer or the like is directed on an unknown sample to generate a resulting spectrum. The resulting spectrum can then be compared with a known spectrum to determine various characteristics of the unknown sample such as its chemical composition.
As is known in the art, it is critical that any deviations in wavelength and/or the instrument's response to light intensity be accounted for to yield accurate analytical results. If these deviations are not accounted for, the generated spectra will not be representative of the sample but will be attributable, at least in part, to these deviations. As a result, the response of the instrument will be mischaracterized and its performance will be flawed.
Various methods of recalibrating optical instruments have been developed in an attempt to account for deviations in wavelength and response to light intensity. One such recalibration method uses calibration standards that are representative of the population of unknown samples. For example, if wheat samples are to be analyzed for protein content, the calibration standards would be a set of wheat samples with known protein contents. When recalibration of the optical instrument is necessary, one or more of the known samples are reanalyzed and the resulting spectra are compared to the standard spectra from the known samples. The instrument response is then recharacterized such that the spectra from the reanalyzed standards match the original spectra for the standard samples.
One problem with such a recalibration method is that the set of calibration standards (e.g. wheat samples with known protein contents) can change and degrade over time. As a result, the sample effect will be confounded with the instrument effect such that the spectrum generated will not accurately reflect the instrument response. To avoid using degraded samples, it is possible to reanalyze the standards or prepare new standards each time the instrument is recalibrated. These approaches, however, are time consuming and introduce operator variability in reanalyzing or preparing the samples.
An alternative to the recalibration method using representative samples from the population, is to use an etalon as the known sample. Generally, an etalon consists of two parallel surfaces where both surfaces have partial reflection and partial transmission of light. For example, a solid block of germanium in air or two spaced, parallel silver plates are etalons. The only requirement is that the instrument must respond to the etalon in a way that allows for recalibration.
Examples of laser systems that utilize etalons to recalibrate instrument response can be found in U.S. Pat. No. 4,241,997 (Chraplyvy) and "Wavenumber Calibration Of Tunable Diode Lasers Using Etalons", Applied Optics, Vol. 17, No. 6, Mar. 15, 1978. These systems, however, disclose the use of an etalon only to recalibrate wavelength and do not address the problem of recalibrating other spectral features such as light intensity. Moreover, to use these recalibration systems, the sample must be removed. In many applications removing the sample is difficult and time consuming.
Thus, an improved method and apparatus for recalibrating optical instruments is desired.